1. Field of the Invention
The present invention relates to gas flow sensors, or exhaust gas mass flow sensors using thin or thick film resistor heaters, otherwise known as hot film anemometers. More particularly, the invention relates a gas mass flow sensor system and the self-calibration of gas mass flow sensors.
2. Description of the Related Art
Various applications require measurement of the mass flow rate of a gas or a mixture of gases at ambient or elevated temperatures. In particular, automotive applications measure exhaust gas mass flow rates or fresh air mass flow rates, for example in engine control.
Hot film anemometers are often used in gas flow measurement equipment, wherein they are suitably packaged within a protective housing, and placed in a gas exhaust pipe, or the like, within the gas flow path a definite direction, typically either parallel or perpendicular or at any angle to the gas flow. Previously, gas flow rates have been determined by first measuring an anemometer's resistance, then estimating its temperature from the resistance, and measuring its voltage or current to calculate velocity or average mass flow rate of the gas. However, certain problems arise when measuring the mass flow rate of hot exhaust gas whose temperature and density varies with engine performance and other operating conditions. Specifically, since exhaust gas temperatures vary over different operating ranges, error may be introduced into the gas flow rate measurement.
A variety of problems exist with conventional gas flow rate measurement techniques. For example, anemometers have been known to degrade in the exhaust gas environment over time, due to thermal cycling and soiling by dust transported by the exhaust gas. Because of this degradation, the heat transfer coefficient of the anemometer varies over time and introduces error into the gas mass flow rate measurement. Also, since an anemometer is present in a harsh and elevated temperature environment, electronic components attached thereto are placed outside this harsh environment and are connected to the anemometer via connection cables or the like. Variations in resistance of the anemometer and the connecting cables, especially during equipment changes, introduce error into gas flow rate measurements. That is, the anemometer and any connected electronics are not interdependent. Furthermore, mounting and orientation of the anemometer, i.e. in parallel to or in perpendicular or in any other fixed angle to gas flow, may also introduce error if it is not mounted in a gas tight, leak proof manner in its harsh, high temperature and high pressure environment. Another problem is that since the exhaust gas may comprise a mixture of different gases such as CO, CO2, oxides of nitrogen (NOx), HC, carbon soot, particulates, water, oxides of sulfur (SOx), and the like, and have concentrations which vary by engine type, engine performance, fuel quality and operating conditions, the density of the exhaust gas varies and may cause errors in the gas mass flow rate measurement. Furthermore, errors in gas mass flow rate measurement may occur due to variation in diameter of exhaust gas pipes or manifolds and the location of the anemometer in the exhaust gas pipe.
The present invention provides a gas mass flow sensor system and a self-calibration method to overcome the problems of conventional sensors, particularly those caused by degradation due to long term use in harsh, corrosive, high temperature, high pressure, varying gas density exhaust gas environments.
It has been found that accurate calculation of gas flow rate can be achieved by determining the power dissipated in a gas flow sensor, rather than its voltage or current. This is because, under certain conditions, power dissipated in the gas flow sensor may be independent of the gas flow sensor's resistance, as well as the resistance of any cables or connections between the gas flow sensor and other components of a gas flow sensor system.
Also, power dissipated by the gas flow sensor varies with gas flow rate and gas temperature. Thus, when the gas flow sensor is maintained at a predefined differential temperature, as compared to the temperature of the gas or other medium, the power dissipated by the gas flow sensor is proportional to the gas flow rate. Thus, to reduce error in gas flow rate measurement, the present invention uses this method to determine power use or power dissipated by of the gas flow sensor, independently of its resistance.
Additionally, degradation of a gas flow sensor causes changes in heat transfer coefficient (h) of the sensor over time, resulting in changes in power dissipation of the gas flow sensor. The gas flow sensor system of this invention applies experimentally established correction factors to the gas flow sensor, to compensate for changes in the heat transfer coefficient (h) caused by degradation. This self-calibration offsets the adverse effects of use, aging, and the like of the gas flow sensor, thus reducing errors in gas flow measurement.